The present invention relates to thermographic recording materials whose prints have improved archival properties.
Thermal imaging or thermography is a recording process wherein images are generated by the use of thermal energy. In direct thermal thermography a visible image pattern is formed by image-wise heating of a recording material.
U.S. Pat. No. 5,672,560 discloses a heat-sensitive imaging material comprising: a support and a heat-sensitive imaging layer formed thereon, the imaging layer comprising a color-forming amount of a substantially colorless, finely divided solid noble metal salt of an organic acid; an organic reducing agent that under conditions of heating is capable of a color-forming reaction with said noble metal salt, thereby producing a colored image; an image toning agent; and a stabilizer compound that mitigates the formation of non-imagewise background color in said imaging layer, said stabilizer compound having the formula: 
wherein Z1, Z2 and Z3 each independently represents hydrogen, an alkali metal ion, an alkyl group comprising 1 to about 8 carbon atoms, an aralkyl or cycloalkyl group comprising 5 to about 10 carbon atoms, or an alkyl group comprising 6 to about 15 carbon atoms; or Z1 and Z2 together represent a divalent alkaline earth metal ion, a divalent alkylene group comprising 2 to about 8 carbon atoms, or a divalent aryl group comprising 6 to about 30 carbon atoms; with the proviso that, when Z1 and Z2 together do not represent a divalent alkaline earth metal ion, at least one of Z1, Z2 and Z3 represents hydrogen or an alkali metal ion.
In printing with thermographic materials for medical applications for viewing with a light box, optimum diagnosis requires a blue-black image tone so that the higher ability of the human eye to distinguish detail with such image tone can be exploited, thereby improving the diagnostic value of such prints. Such image tone should be independent of the shelf-life of the thermographic recording material prior to printing and also of archival time after printing. Image tone can be assessed on the basis of the L*, a* and b* CIELAB-values as determined by spectrophotometric measurements according to ASTM Norm E179-90 in a R(45/0) geometry with evaluation according to ASTM Norm E308-90. Ingredients are required for substantially light-insensitive thermographic recording materials, which provide a balance of image tone stabilizing properties, enabling prints to be obtained with image tone which after pretempering is less dependent upon storage time prior to printing and to archival time after printing, while exhibiting image tone acceptable for radiologists viewing images in transmission on a light box.
It is therefore an aspect of the present invention to provide a substantially light-insensitive black and white thermographic recording materials with improved image tone stability not only capable of producing prints with image tone which is less dependent upon storage time prior to printing.
It is therefore a further aspect of the present invention to provide a substantially light-insensitive black and white thermographic recording materials which is less dependent upon archival time after printing, while having an acceptable image tone for medical and graphics images.
Further aspects and advantages of the invention will become apparent from the description hereinafter.
It has been surprisingly found that substantially light-insensitive thermographic recording materials comprising a thermosensitive element containing particular phosphonic acid derivatives are capable of providing prints whose image tone is less dependent upon storage time prior to printing and is less dependent upon archival time after printing, such a balance of properties combined with acceptable image tone not being provided by prior art thermographic recording materials.
Aspects of the present invention are provided by a substantially light-insensitive black and white thermographic recording material comprising a thermosensitive element and a support, the thermosensitive element containing at least one substantially light-insensitive organic silver salt, an organic reducing agent therefor in thermal working relationship therewith, an image tone stabilizer and a binder, wherein the image tone stabilizer is a compound according to formula (I): 
wherein R1 is hydrogen, an alkali metal ion, an ammonium ion, a phosphonium ion, a sulphonium ion, an alkaline earth ion or an optionally substituted alkyl, cycloalkyl, heterocyclic, alkaryl, aralkyl, heteroaryl or aryl group; R2 is an optionally substituted alkyl, cycloalkyl, heterocyclic, alkaryl, aralkyl, heteroaryl or aryl group; or R1 and R2 together represent the atoms necessary to form a ring structure; and X is hydrogen, an alkali metal ion, an ammonium ion, a phosphonium ion, a sulphonium ion or an alkaline earth ion.
Further aspects of the present invention are provided by a recording process comprising the steps of: (i) bringing an outermost layer of a thermographic recording material as described above into proximity with a heat source; (ii) applying heat from the heat source imagewise to the thermographic recording material in a substantially water-free condition while maintaining proximity to the heat source to produce an image; and (iii) removing the thermographic recording material from the heat source.
Use of a compound according to formula (I): 
wherein R1 is hydrogen, an alkali metal ion, an ammonium ion, a phosphonium ion, a sulphonium ion, an alkaline earth ion or an optionally substituted alkyl, cycloalkyl, heterocyclic, alkaryl, aralkyl, heteroaryl or aryl group; R2 is an optionally substituted alkyl, cycloalkyl, heterocyclic, alkaryl, aralkyl, heteroaryl or aryl group; or R1 and R2 together represent the atoms necessary to form a ring structure; and X is hydrogen, an alkali metal ion, an ammonium ion, a phosphonium ion, a sulphonium ion or an alkaline earth ion; in a substantially light-insensitive thermographic recording material as an image tone stabilizer is also provided by aspects of the present invention.
Further advantages and aspects of the present invention will become apparent from the following description.